This are probably the most commonly encountered
numbering system in the UK - and the most informative. It consists of two or
more letters followed by a number (normally two digits). Examples - UL41, ECC85,
UABC80.

The first letter gives heater rating.

Character

Heater Rating

A

4V

B

180mA

C

200mA

D

0 - 1.5V (previously 1.4V)

E

6.3V

F

12.6V

G

Misc. (previously 5V)

H

150mA

K

2V

L

450mA

P

300mA

T

7.4V

U

100mA

V

50mA

W

600mA

X

450mA

The remaining letters give the types of device in the valve. They
are normally listed in alphabetical order.

Character

Device Type

A

Signal Diode

B

Double Diode

C

Signal Triode

D

Power Triode

E

Signal Tetrode

F

Signal Pentode

H

Hexode or Heptode (Hexode type)

K

Octode or Heptode (Octode type)

L

Output Tetrode or Pentode

M

Magic Eye (Tuning Indicator)

N

Gas-filled Triode (Thyrathon)

Q

Nonode

X

Gas-filled Full-wave Rectifier

Y

Half-wave Rectifier

Z

Full-Wave Rectifier

The first digit indicates the base type. Where
there is only one digit this is assumed to be the second digit, and be preceded
by a zero. For example, EM4 should be interpreted as EM04.

Digit

Base Type

0 and 1

Miscellaneous Bases (P-Base, Side Contact etc.)

2

B10B (previously B8B/B8G (Loctal))

3

International Octal (8-pin with centre locating spigot)

4

B8A (8 pin with locating pip on side)

5

B9G and B9D (wire ended)

6 and 7

Subminiatures

8

B9A (9-pin glass)

9

B7G (7-pin glass)

The remaining digit(s) are used to differentiate
between valves that would otherwise have identical numbers:-

One digit for early valves

Two figures for later entertainment valves

Three or Four figures for later professional types

GEC Code (also used on Marconi and Osram valves)

This consists of one or two letters followed
by a number (normally two digits). Examples L63, KT88.

The letters have the following meaning.

Characters

Device Type

A

Industrial Valve (could be anything!)

B

Double Triode

D

Diode

GU

Gas-filled Rectifier

GT

Gas-filled Triode (Thyrathon)

H

Signal Triode (High Impedance)

H

Signal Triode (Medium Impedance)

KT

Kinkless Tetrode (Beam Tetrode)

KTW

Vari-mu RF Kinkless Tetrode

KTZ

Sharp Cut-off RF Kinkless Tetrode

L

Signal Triode (Low Impedance)

MU

Indirectly Heated Rectifier

N

Output Pentode

P

Output Triode

PX

Output Triode

QP

Quiescent Push-Pull Double Pentode

S

Tetrode

U

Rectifier

VS

Vari-mu Tetrode

W

Vari-mu Pentode

X

Triode-Hexode, Heptode, Octode (Frequency Changer)

Y

'Magic Eye' Tuning Indicator

Z

Sharp Cut-off HF Pentode

The digits are simply to distinguish similar
valves and cannot be decoded:-

One digit for early valves

Two figures for later valves

Note: Suffix 'M' indicates external metallising

Mazda Code

These can be confused with the US code (below).
The codes consist of digits, then letters, and then digits. Examples 10D2, 6F18.

The first digits give the heater rating.

Digits

Heater Rating

1

1.4V

6

6.3V

10

100mA

20

200mA

30

300mA

The letters indicate the type of valve. Mazda
codes do not normally double-up the letters - so for example 'D' would be used
for single and multiple diodes.

Characters

Device Type

C

Frequency Changer

D

Signal Diode

F

Signal Tetrode or Pentode

K

Gas-filled Triode (Thyrathon)

L

Signal Triode

M

'Magic Eye' Tuning Indicator

P

Output Tetrode or Pentode

U

Half-Wave Rectifier

UU

Full-Wave Rectifier

The final digits distinguish between valves that
would otherwise have identical codes.

Miscellaneous Early British Codes

A selection of some of those codes from the era
of British 4, 5 and 7-pin based valves, and also Mazda Octals. Some codes were
used by one manufacturer only, some by more than one.

I have tried to limit this table to those codes
that, in general, always had the same meaning, though some had different shades
of meaning under different brand names. Sometimes code letters were combined
to identify a multiple valve, for example the AC/2PenDD, a double-diode output
pentode with 4V heater from Mazda.

Characters

Device Type

AC

4-volt Heater

D

Single or Double Diode

DD

Double Diode

DDT

Double-Diode Triode

FC

Frequency Changer

H

High-Impedance Triode

M

4-volt Heater

ME

'Magic Eye' Tuning Indicator

Pen

Output Pentode

PM

Philips/Mullard

PP

Power (Output) Pentode

PT

Output Pentode

R

Full-Wave Rectifier

SP

Straight RF Pentode

TH

Triode-Heptode or Triode-Hexode

TP

Triode-Pentode

U

Rectifier (usually Half-Wave)

UU

Full-Wave Rectifier

VP

Vari-mu RF Pentode

US Code

The US codes consist of digits, then letters,
the digits, then possibly further letters. Examples 6V6GT, 5Z4G.

The first digits give heater voltage rating,
with the exception that '7' and '14' are used to indicate 6.3V and 12.6V valves
(respectively) with Loctal bases.

The next letters indicate the type of valve,
but there is no real consistency of coding. 'S' often indicates a single-ended
(no top cap) version of an earlier valve with such a cap.

The second digits give either the number of active
electrodes, or the number of external connections. Again though this does not
always work out!

The final letters often specify the type of envelope.

Characters

Envelope

G

Large Glass Envelope

GC

Glass Compact

GT

Glass Tubular

M or None

Metal Envelope

WA

High Quality Version

Chas Miller commented:

There never has been a suffix "M" for a
metal tube. All the original octal tubes were metal cased and their nomenclature,
e.g., 6K8, 6K7, 6Q7, 6V6, 5Z4, etc., indicates this. It was only when glass
versions were produced that the suffixes "G" (glass) and "GT" (glass, tubular)
appeared. I have a painful recollection of writing an article for Practical
Wireless some years ago concerning a military receiver that used metal valves.
I referred correctly to these throughout as 6K8, 6K7, etc., but the then editor
in his wisdom decided to add the erroneous "M" suffix, resulting in my receiving
scathing letters from readers.

It has long been a cherished myth that the last
digit in tube numbers represents either the number of electrodes or the number
of external connexions. One has only to glance through a tube data book to
dismiss the idea. For instance, the 5Y3GT and the 5Z4G are both full-wave
rectifiers with four external connexions (but the first has five base pins
and the second eight). The 6SL7GT is a double triode (i.e., 2 x three electrodes)
but has eight base pins. You can find plenty more examples.

Loctal valves. Those with the first digit
"7" originally were rated at 7V on the heaters, and those with the first digit
"14" at 14V. These voltages were chosen to correspond with those of nominally
6V and 12V automobile storage batteries whilst on charge from a dynamo. This
remained for only a few years before the ratings were changed to 6.3V and
12.6V respectively.

Norman Leal disagrees:

Here in the US there were metal tubes with
an "M" after the number, 6A8M, 6L7M, even "MG", 5Z4MG. These where larger
versions of the small metal tubes. In some cases a metal shield was actually
built right over a "G" type tube. These tubes were also built in other countries.
I've attached a picture of a 6A8M tube made in France.

So does Rob Jones:

Regarding Chas Miller's comments at the
end of this listing, his paragraph 2 is misleading. The second figure group
of a valve type No in the American RETMA designation system refers to the
number of ACTIVE ELEMENTS in that valve. Note that in the case of a metal
valve, the outer shell is counted as an active element.

6K8: Triode hexode frequency changer
- 8 active elements: heater, common cathode, 2 control grids, hexode g2/g4
(commoned), hexode g3, and 2 anodes. This is a glass valve, if there is a
metal version, it will not have a second figure group no 8.

A number of Military codes are used, but these
are unlikely to be encountered in domestic receivers. If one is encountered
the best course of action would probably be to find out the commercial equivalent
from a suitable data book or web site - or from my Valve
Data CD-ROM!

Military codes cannot decoded in any way - they
are generally a fixed prefix (such as CV) followed by a serial number.

The information on this page has been gathered
and assembled from various sources including "Radio Bygones" magazine
No. 9 (February/March 1991). Corrections and additions welcomed!